Flux pump with thermal cryotrons



Aug. 4, 1970 T. A. BUcHHoLD 3,522,512

FLUXk PUMP WITH THERMAL CRYOTRONS Filed Sept. 15, 1967 3 Sheets-Sheet 1fr? Ve 02".' Theodor A. uc/m/a by Kum H/ls Aorney: y

ug- 4, 1970 T. A. BucHHoLD :5,522,5L2fy FLUX PUMP WITH THERMAL CRYOTRONSY 3 Sheets-Sheet 2 Filed Sept. 15, 1967 fn Ver; t; or.' 777e odor Aac/'lo/d, bym www H/ .s A orney Aug# 4, 19,70 l T. A. BucHHoLD 3,522,512

FLUX PUMP WITH THERMAL CRYOTRONS Filed sept. 15, 1967 :s sheets-sheet sy (A) c ...VM/Alli J ,0f/,4.5i y (c) Jli/FIR l da I i A fnl/anton'. 777eoder A. ac/)o/d.

United States Patent O 3,522,512 FLUX PUMP WITH THERMAL CRYOTRONSTheodor A. Buchhold, Schenectady, N.Y., assignor to General ElectricCompany, a corporation of New York Filed Sept. 15, 1967, Ser. No.667,934 Int. Cl. H02m 7/00; H03k 3/38 U.S. Cl. 321--8 8 Claims ABSTRACTOF THE DISCLOSURE A thermal cryotron is made by sandwiching layers ofmaterials such as a heated resistive tape having on both of its sides abilar niobium-zirconium tape and on the outside of each of theniobium-zirconium tapes an insulating material and a conductive metalstrip may be placed (on the outside lof the layers). A system forutilizing this switch in a flux pump is described.

My invention relates to an electric flux pumping arrangement andparticularly to an electric flux pump in which switching is effected bya thermal cryotron.

In my copending application, Cryogenic Pumped Rectier Systems, Ser. No.642,625 iled May 2, 1967, now Pat. No. 3,356,924, and assigned to theassignee hereof there is disclosed an electric cryogenic llux pumpingarrangement and electrical control apparatus for use therewith. Thesystem described therein is particularly concerned with the applicationof superconductive devices maintained at low temperature environments inthe neighborhood of -15 Kelvin. Specifically, the application disclosesan arrangement including a superconducting component and asuperconducting source of varying electrical potential. Losslesssuperconducting paths are provided for alternately coupling thesuperconducting lossless source of varying electrical potential in anelectric circuit relationship With a superconducting component. Thearrangement is completed by switching means which are controlled to givecertain switching results and which are connected in each 4of thealternate superconducting paths for alternately switching thesuperconducting source of varying electric potential in and out ofelectric circuit relationship with the superconducting component throughalternate ones :of the superconducting paths in proper phaserelationship with the varying electric potential supplied from thelossless superconducting source. The switching means of this device arecontrolled by special sensing devices which assure that the switchingoff occurs at a very small current and in proper time relationship.

My invention concerns itself with a new arrangement of such switchingmeans and the control circuit. In the former invention as set forthabove, the switching is effected by magnetically controlled cryotrons.However, magnetically controlled cryotrons are expensive, relativelylarge and the required electronics is not simple. For example, niobiumwhich is commonly used in these cryotrons has a resistivity of about 1106 ohms centimeter. Niobium with 25% zirconium, of this example, has aresistivity of about 25x10F6 ohms centimeter. The 75% niobium-25%zirconium mixture cannot be controlled magnetically since the requiredcontrol fields are too high. Niobium-zirconium, which has a transitiontemperature of 10.5 Kelvin, is particularly attractive for control byheating. Nb (52%) and Ti (548%) has similar properties and is alsosuitable for use in a heating cryotron. When the flux pump of theapplication referred to above is provided with a heating cryotron, thecryotron control becomes simpler and higher eiciency can be obtained.The simplicity lpossible with such device becomes apparent inconsidering applicants present invention where switch- Cri 3,522,512Patented Aug. 4, 1970 ICC ing is accomplished without the necessity ofelectronic apparatus in the switching control circuit.

It is an object of my invention to provide a ux pump system and athermal cryotron of special properties.

It is another object of my invention to provide a flux pump circuit foruse with a thermal cryotron.

It is another object of my invention to provide a thermal cryotronhaving a small time constant and having a high reistance at a lowtemperature and zero resistance at a still lower temperature.

In brief, my thermal cryotron is made of a material which changes from anon-resistive to a resistive state at a low temperature, an example ofsuch a cryotron being niobium-zirconium tapes arranged in bilarrelationship with a heating element of nickel chrome between them. TheNiCr heating element has a very thin lm of electric insulating materialabout it. This cryotron may be sandwiched between two copper plateswhich have approximately the temperature of liquid helium. Between thecryotron and the copper plate is a thin layer of insulation with athickness determined by the thermal time constant desired. In operation,the bilar cryotron tapes of niobium-zirconium are heated by the heatingelement to a point where they become resistive. The tape is madebifilar, that is doubled back upon itself because for ellicientoperation the total heat output of the heating element must be absor-bedand the heat lost must all pass through the layer of insulation whichhas a critical thickness to allow only a certain amount of heat to passunder given conditions.

Other objects, features, and many of the attendant advantages of thisinvention will be appreciated more readily as the same becomes -betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings wherein likeparts in each of the several gures are identified by the same referencecharacter.

FIG. 1 shows a thermal cryotron.

FIG. 2 shows one embodiment of a control circuit of a flux pump using athermal cryotron.

FIG. 3 is a series of current versus time characteristic curves of thecircuit represented in FIG. 2 and illustrates the mode of operation ofthis circuit.

FIG. 4 is a hysteresis loop showing the mode of operation of thesaturable reactor.

FIG. 5(11) shows the sharp transition point of a typical thermalcryotron with resistance as a function of the temperature while FIG.5(b) shows the relationship between temperature, heat generated in theresistive phase, and current for a given thermal switching material.

FIG. 6 shows an improved phase shifting circuit to vary the outputvoltage of the flux pump from positive to negative and vice versa usingthe same basic circuit as FIG. 2.

FIGS. 7(11), (b), and (c) shows current and voltage versus timerelationships at various points in the apparatus represented in FIG. 6While FIG. 7(d) shows a hysteresis curve according to the mode 0foperation of the saturable reactor of FIG. 6.

FIGS. 8(a), (b), and (c) show a phase shifting circuit for operatingunder reduced power and voltage.

FIGS. 9(a), (b), and (c) show a circuit with switch (in realityelectronic switches) and hysteresis curves which can operate underreduced power and voltage.

The thermal cryotron assembly 1 shown in FIG. l has a central heaterelement 2 made of a material, for eX- ample, nickel chrome, whichremains resistive at very low temperatures so that when an electriccurrent ilows through this element, heat is generated. The heatermaterial has a high resistance at very low temperatures so that the bodyof the heater is small and has as small a heat storage capacity aspossible to provide high switching speed. This very thin element 2 isshown divided into two parts, electricity flowing in one direction inone of the parts and in the opposite direction in the other part. Eachof the parts is coated with a very thin layer 3 of electrical insulatingmaterial. The thermal cryotron tape elements 4 conduct the electricalcurrents which are to be switched on or off, and are made of a materialhaving a resistivenonresistive transition temperature at some very lowtemperature, for example, 10.5 K. These tapes are of a single tapefolded upon itself to accomplish full control of the heat generated bythe heater.

Other arrangements of cryotron tape and heater are possible, forexample, the cryotron tape may be folded several times and the singleheater element placed in the middle of the folds.

The cryotron elements 4 must have as fast a thermal time constant aspossible to allow fast switching. Thermal time constant may be definedby the expression:

where V is the volume of the element; c is the specific heat of materialper unit volume; sis the surface area; and is the thermal conductivityof the insulating material. To effect the desired small time constantthe cryotron tape must have a high resistance in its resistive conditionso that it may be small in length and must be exceedingly conductive inits conductive state so that it may have a small cross section and asmall volume. The transition point of the material should be at a lowtemperature because c in the formula above is proportional to the thirdpower of the absolute temperature of the transition point. In thisembodiment the cryotron tape is made of niobiumzirconium in a 75% to 25%ratio, although any material having the desired characteristics may beused. For example, the cryotron tape may be made of niobiumtitaniumalloy. The tape or elements described above comprise the operativecryotron element itself. (The parts in the drawing of FIG. 1 are notshown to scale but for' purposes of illustration only.)

The dimensions of the elements of the cryotron should be kept as smallas possible so that the amount of heat exchange necessary to go fromsuperconductive to resistive and vice versa is as small as possible.Consequently, for given external conditions the smaller the elements thefaster the cryotron switching because less heat addition or subtractionis necessary. In order to apply the thermal cryotron in the ux pumparrangement, two elements are added: a cooling material and a pair ofthermal insulating plates 5 of critical heat conductivity which areplaced between the cryotron tape and the cooling material as shown inFIG. 1. A cooling plate 6 may be inserted be tween the heat insulation 5and the cooling material or may be omitted if desired, and the coolingmaterial of liquid helium (not shown) which cools the plate in thisembodiment may be applied directly against the thermal insulatingmaterial. The thickness of the thermal insulating material is determinedby the losses and the rate at which heat is to be conducted from thecryotron tape 4 to the cooling plate 6 or to the liquid helium as thecase may be. It is readily seen from the drawing of FIG. l that theamount of heat applied to the cryotron tape can be varied by varying theelectric current which is passed through the nickel-chrome heaterelement. The rate of cooling of the cryotron tape 4 is controlled by thethickness of the thermal insulating material 5 between the tape and thecopper cooling plate or the liquid helium depending on the constructionof the apparatus. In the case under consideration the transitiontemperature is about 10.5 K. and liquid helium, which has a boilingpoint of 4.2 K., is the preferred cooling material. As can readily beseen, if other cryotron materials having other characteristic transitiontemperatures are used other refrigerants or cooling materials may beused.

After the cryotron tape is made resistive by the heater, the heatercurrent is interrupted and the back current flowing through the cryotronproduces suicent heat to maintain the cryotron tape in its resistivestate. The heat losses are minimized to avoid unnecessary evaporation ofliquid helium. At the same time the thermal time constant must be keptsmall and to effect this result the volume of the tape is kept as smallas possible.

A simplified rectification circuit is seen in FIG. 2. The powertransmission elements and the coil are superconductive at all timeswhile the cryotron is superconductive part of the time and the heaterelement is not superconductive. An alternating current square Wavegenerator having a low frequency on the order of 10 cycles persecond isapplied across the transformer which has a split secondary. Tapped fromthe center of the secondary winding 8 is a coil 10 which is energizedwhen the circuit is in operation. The transformer with its windings 7and `8 are inside the low temperature area. Electr-ically connected toeach end of the secondary winding 8 is a bifilar cryotron tape 4.Electrically connected to the other end of each cryotron tape is asaturable core winding 9 which in turn is electrically connected to thefree end of coil 10. Electrically connected in parallel across eachsaturable core winding 9 is a series circuit comprising rectifier 11, avariable resistance 12, and heater element 2. The heater 2 is locatedadjacent the cryotron tape 4 in the manner shown in FIG. l while therectifier 11 and variable resistance 12 are located outside of thecryogenic temperature zone and are at room temperature so that theresistance may be Varied to obtain the desired operating conditions forthe heater and to prevent heat losses from the rectifier and resistancefrom evaporating the liquid helium. That portion of the flux pumpingsystem which operates at cryogenic temperatures is shown enclosed by thedash-dot line which represents a suitable cryogenic housing. Each halfof winding 8 is connected in an identical circuit to obtain push-pulloperation as flux builds up gradually in the cryogenic central coil 10.

The operation of the device of FIG. 2 can be explained by the curves ofFIGS. 3(a), (b), and (c). When cryotron tape 4 is superconducting andcryotron tape 4 is resistive, a current I1, as shown in FIG. 3(b), ovsthrough tape 4 and saturable core 9 operates at saturation point 14shown in FIG. 4. The heater 2 begins to operate as soon as the currentI1 starts to become negative and the entire voltage is across thesaturable core Winding 9. At this time the cryotron 4 is stillsuperconductive. Therefore, current passes through the rectifier 11 andheater 2. At this point the current AI flows through the heater 2causing heat to be given out from the heater to the adjacent cryotrontape 4. The transition temperature is reached after time At and cryotron4 becomes resistive and the transformer voltage is now across cryotron 4so that a small negative current flows through 4 because the saturablecore 9 has reached negative saturation and the voltage at the core 9 andthe current AI has become zero. It is at this point that the exactamount of the insulation of the cryotron becomes important because thecurrent i must produce the right amount of heat so that the cryotrontape temperature is kept approximately constant and at a point justabove the transition temperature. When the alternating current voltagereverses, the current i will become positive and equal to the smallmagnetizing current im because the rectifier prevents the current fromowing through the heater. Since the current im -is not sufiicient toprovide transition temperature, the cryotron tape Will cool and becomesuperconductive. Thus, cryotron tape 4 is again superconductive and thecycle is completed. It is noted that the circuit embodied in FIG. 2works to rectify the square wave input but will not extract power fromthe field surrounding the flux coil and will not allow regulation of thevoltage across the flux coil.

A similar cycle is taking place in the other half of the circuit withcurrent I2 building up in cryotron 4. The electric currents I1 and I2are actually many times the value of back current i.

The resistance-superconductive transition point is shown at in FIG.5(a). In this figure the plot of resistance and temperature is shown andthe transition point 0 is indicated on the temperature axis. FIG. (b)shows a graph of thermal transfer rate against temperature with theheavy line indicating the thermal transfer rate through the Iinsulationof the cryotron. At current i2 the transition temperature is reached, athigher current i3 the transition temperature is exceeded and at lowercurrents the cryotron transition temperature cannot be reached.

It is readily apparent that the cryotron arrangement of FIG. 2 hascertain limitations. For example, this arrangement can only rectify andcannot extract power from the buildup liux field. An arrangement forovercoming some of these limitations is shown in FIG. 6.

The circuit of FIG. 6v permits the direct current output voltage to thecoil to be regulated either positively or negatively. By regulating thevariable phase shifter so that the negative part of the voltage curvesof FIG. 7 is larger than the positive part, energy is extracted from thefield of the coil 10 and returned to the power source.

For a detailed explanation of the influence of phase shifting on directcurrent output see the article of T. A. Buchhold in Cryogenics of August1964, p. 215, FIG. 6 and the explanatory material. Further descriptionis to be found in application Ser. No. 642,625 referred to above.

The flux pumping arrangement shown in FIG. 6 is in its essentials,similar to that of FIG. 2 with the addition of the electron controlcircuitry. FIG. 6 has the following changes from FIG. 2. The saturablecore winding of FIG. 2 is now replaced by a saturable core transformerwinding composed of a primary saturable core winding having twosecondary windings 16, 17 arranged as shown in FIG. 6. The upper of thetwo saturable core n secondary windings 16 matches its voltage to theheater and the lower is a retarding coil or winding.

The operation of the variable phase shifter in the apparatus of FIG. 46is essentially the same as that shown in FIG. 18 of application Ser. No.642,625 referred to above and this mode of operation is discussed in thearticle, superconductive Power Supply" for the ux pump starting on p.213 of the Cryogenics Magazine of August 1964. Specifically, theresultant currents passing through the halves of the secondary winding 8of the transformer of FIG. 6 are shown in (b) and (c) of F-IG. 7.

All elements within the dotted line are in a cryogenic temperatureenvironment.

FIG. 7(a) shows the voltage and current effects of applying a phaseshift. The operation of FIG. `6 shown in FIG. 7 as follows: The currentI2 flows from time t2 to time t5, shown in FIG. 7(c), and the voltage V2through this time is positive and then negative as shown in FIG. 7(a).Current indicates the current through the resistive cryotron 4 and thisis first negative and then positive when the voltage reverses. Thepositive voltage tends to reduce the cryotron current i to the smallerpositive magnetizing current im, which would make cryotron 4superconductive. This is avoided by a retarding current iR in the coil17, i.e., the lower half of the saturated secondary coil. This retardingcurrent -fiows between indicated times t2 and t4. The retarding currentiR is obtained when the amplifier A1 closes the electronic contact 20 atthe time t2 when the coil 16 produces a voltage to amplifier A1.Retarding coil or winding 17 is connected to a positive voltage source21 through the amplifier switch 20. This switch is closed until time t4when the phase shifter produces a pulse 22 shown in FIG. 7(11). Pulse 22causes the amplifier to open the electronic switch stopping current iRfrom flowing through retarding coil 17 and reducing the current to im.The cryotron 4 then goes superconductive as soon as enough time haselapsed for it to cool past the transition point.

In the event that the positive magnetizing current going through thesaturated windings is too large, it may be 6 reduced by a small currentcoming from a negative battery 23.

While in FIG. 6 the heater voltage is supplied by the winding 16, it isclearly possible that winding 16 could be connected to an amplifierwhich in its turn would energize the heater toy the desired temperature.

The electrical circuits described in FIGS. 2 and 6 will operate to bestadvantage only at full power and direct current voltage. However, underoperating conditions it may be necessary to use reduced power andreduced direct current voltage. This would cause considerable loss inefficiency in the circuits referred to above.

When the voltage is reduced, the time constants of the saturable reactorgive rise to delays in saturation reaction time which cause the directcurrent voltage to decrease much tfaster than the applied alternatingcurrent voltage. Described below are two electrical circuits forobviating this difficulty at certain levels.

An electrical circuit which is adapted for use with a small alternatingcurrent voltage is shown in FIG. 8 and has only one saturable reactor 9.The operation of this circuit is explained as follows, starting with anegative voltage at the right side of the primary of coil 9 the flux 1pwill change along C-d as shown in FIG. 8(b) and the coil secondarywinding 24 will cause currents to flow in the series circuit of which itis a part. This electric current in passing through heating resistor 2Will cause the cryotron tape 4 to become resistive. The cryotron tape 4becomes resistive, for example, at point d of FIG. 9b and develops atits terminals a voltage which causes amplifier A3 to close the switch 25thereby bypassing the secondary winding 24 of saturable transformer 9.Since the saturable core is short circuited, its flux will not changeuntil the phase shift circuit opens the switch and the flux increases upto positive saturation.

The heater cryotron has the characteristic that with a smaller Voltageon it its resistance decreases about linearly and therefore its losses.Considering the case of which a smaller alternating current voltage isapplied, then the series resistor 26 is varied to give a total smallerresistance in the control circuit and the cryotron heating resistor 2heats in about the same time even rwith less applied Voltage. Theoperation works in a similar manner as the other embodiments in that thecryotron tape 4 goes resistive, the amplifier A3 closes the switch 25.The switch 2S will stay closed until the phase shifter opens the switch25 and the fiux moves back from d2 to saturation. Because of the smallervoltage, d2 is closer to saturation than d in FIG. 8(b). The advantageof this circuit is that the time needed to reach saturation is now aboutthe same as in the previous two circuits since the core voltage isproportionally smaller. The circuit has the advantage that with smallervoltage the cryotron losses decrease about linearly.

Another circuit which will operate under reduced power and reduceddirect current voltage is shown in FIG. 9. The circuit shown in FIG. 9is shown without phase shifting but nonetheless may be adapted for phaseshifting if so desired.

This electric circuit is adapted to avoid time loss due to delays incore saturation. The circuit operates efficiently at either of twovoltages or operates at a value equal to the sum of the voltages. Thevalue of these voltages depends upon the value of the saturable reactorsshown in FIG. '9. The electric circuit shows two saturable reactors 27,28 in series, the larger 27 of these reactors may be for about 801% offull voltage and the smaller `28 of the two reactors for about 20% offull voltage. It the total rated Voltage is applied, both reactors |workin series. If a small voltage such as 20% is to be applied then thelarger reactor 27 will have la small positive bias current Ibo as shownin FIG. 9(b) which will keep this core in positive saturation even if anegative back current flows. From this it can be seen that the flux ofthe larger core 27 does not change and only the smaller core iseffective. The electrical circuit shown in FIG. 9 shows a gang switch 29which is closed for low voltage. When this switch 29 is closed, thelarger winding 27 is effectively out of the circuit and only the smallerwinding 28 operates at 20% of capacity. Further, the resistance 30 inthe circuit is lessened by a predetermined amount to allow the cryotronheater 2 to heat the cryotron sufficiently for normal operation. Whenfull or high voltage is used, the double pole single throw switch 29 isopened and both secondary windings are in the circuit giving the effectof a single winding of 100% value. This circuit has also the advantagein having smaller losses at reduced voltage.

In this electrical circuit, the retarding :winding which is needed forphase shifting is not shown, but the circuit is adaptable to thismodification.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A flux pumping arrangement including in combination a superconductinginductance,

a superconducting source of varying electric potential,

alternate lossless superconducting paths for operatively coupling saidsource of varying potential in superconducting electric circuitrelationship with said superconducting inductance,

a thermal cryotron operatively connected in each of said alternatesuperconducting paths for alternately switching said superconductingsource of varying electric potential in and out of superconductingelectric circuit relationship with said superconducting inductancethrough alternate ones of the superconducting paths, and

control means for said cryotron electrically connected to each pathcomprising a rectifier, a variable resistance, a resistance heater, anda saturable core winding in series with each other whereby said heaterresistor is controlled in desired phase relationship with the varyingelectric potential.

2. A flux pumping arrangement as set forth in claim 1 in which saidthermal cryotron comprises a bitilar tape having a lowresistive-nonresistive transition temperature and said resistance heateris located between and in close proximity to parts of said thermalcryotron tape.

3. A flux pumping arrangement as set forth in claim 2 in which saidsaturable core winding is electrically connected by one terminal to saidcryotron tape and its other terminal is connected to saidsuperconducting inductance whereby current flows in said heater onlywhen the current in said saturable `core is in a certain direction.

4. A flux pumping arrangement as set forth in claim 3 in which saidthermal cryotron is immersed in a cryogenic liquid,

and

insulating plates of critical thickness are mounted one lying againsteach outer side of said thermal cryotron tape to allow a certain rate ofthermal conduction of heat away from said cryotron tapes through saidinsulating plates to the cryogenic liquid surrounding said thermalcryotron.

5. A ux pumping arrangement as set forth in claim 3 in which i saidrectifier, said variable resistance, and said resistance heater are inelectrical series with each other and connected in parallel with saidsaturable core whereby said saturable core, said rectifier, and saidvariable resistance form a control circuit for varying the electriccurrent to said resistance heater.

6. A flux pumping apparatus for full or reduced alternating currentvoltage including in combination a superconducting inductance,

a superconducting source of varying electric potential,

alternate lossless superconducting paths for operatively coupling saidsource of varying potential in superconducting electric circuitrelationship with said superconducting inductance,

thermal cryotron tape means operatively connected in each of saidalternate superconducting paths for alternately switching saidsuperconducting source of varying electric potential in and out ofsuperconducting electric circuit relationship with said superconductinginductance through alternate ones of the superconducting paths, and

control circuit means comprising a rst circuit comprising two saturabletransformers having their primary windings in electrical series in eachsuperconducting path and their secondary windings in electrical seriesin said control circuit,

a rectier, a variable resistor having a switch between one end and thevariable tape and a cryotron tape heater in electrical series with saidsecondary windings, and

a second circuit comprising a coil, a resistance, a voltage source, anda second switch in series,

said rst switch and said second switch being mechanically connected sothat both switches may be closed simultaneously when reduced voltage isconducted from said superconducting source.

7. An electrical circuit comprising a superconducting inductance,

a superconducting source of varying electric potential,

alternate lossless superconducting paths for operatively coupling saidsource of varying potential in superconducting relationship with saidsuperconducting inductance,

a thermal cryotron for controlling current ow through eachsuperconducting path, and

thermal cryotron control means for varying the direction of energy flowin said circuit from said source of varying electric potential to saidsuperconducting inductance or alternatively from said superconductinginductance to said source, said thermal cryotron control meanscomprising a saturable core transformer having its primary winding inelectrical series with said superconducting inductance and said thermalcryotron and having two windings in the secondary thereof, a lirstwinding being connected in electrical series with a resistor for heatingsaid thermal cryotron, a second winding for retarding the current insaid irst winding so that the current for heating said thermal cryotronis controlled in a desired phase relationship with said varying electricpotential.

8. A ux pumping apparatus for a full or reduced alternating currentvoltage comprising a superconducting inductance,

a superconducting source 'of varying electric potential,

alternate lossless superconducting paths for operatively coupling saidsource of varying potential in superconducting electric circuitrelationship with said superconducting inductance,

`a thermal cryotron tape means 'operatively connected in each of saidalternate superconducting paths for alternately switching saidsuperconducting source of varying electric potential in and out ofsuperconducting electric circuit relationship with said superconductinginductance through alternate ones of said superconducting paths,

control means for causing said cryotron tape to alternate between its`resistive and non-resistive condition under a variety of levels ofmaximum electric output of said superconducting source,

said control means comprising a saturable transformer References Citedmeans having a primary winding connected in each of said alternatesuperconducting paths, UNITED STATES PATENTS a secondary windingconnected in electrical series with 2,665,884 1/ 1954 EFCSSOII. et al.

a cryotron tape heater, a variable resistor, and a 5 3,054,978 9/1962Schmldllll et al. 338-32 X rectifier and having a switch in parallelwith said 3,059,196 10/1962 Leutz 307-306 secondary winding, 3,356,92412/1967 Buchhold 321-8 a ltlllftonnected across said source of electricOTHER REFERENCES an amplifier mechanically connected to said switch for10 International Advances in Cryogenic IEngineering, opening and closingsaid switch, said amplier being Plenum Press, New York, 1965, TheGeneration of electrically connected to said phase shifter whereby HeavyCurrents in Superconducting Circuits, pp. 98-104. a signal from saidphase shifter causes said amplier to open said switch and said amplifierbeing electri- LEE T- HIX, Primary Examinl cally connected across saidcryotron tape whereby a 15 W H BEHA JK Assistant Examiner voltage acrosssaid cryotron tape causes said arnplifier to close said switch and shortcircuit said US C1. XR,

secondary winding. 307-306; 338-32

